1. Field of Invention
This invention pertains to semiconductor devices and in particular the protection from electrostatic discharge.
2. Description of Related Art
As the level of integration of semiconductor product has grown, device structures have shrunk leaving less volume of material to dissipate heating from current surges caused by electrostatic discharge (ESD). Keeping input and output devices large enough to dissipate the energy from an ESD uses a lot of valuable semiconductor real estate. Various attempts have been made to change the path of the discharge current and provide a way to absorb the energy while allowing the transistor devices to get smaller.
In U.S. Pat. No. 5,416,351 (Ito et al.) a Zener diode is embedded into the drain of a MOS device as an ESD protection device. The ESD protection device provides a low voltage trigger for avalanche breakdown to discharge the ESD current away from the drain. In U.S. Pat. No. 5,446,302 (Beigel et al.) and U.S. Pat. No. 5,637,901 (Beigel et al) a diode connected bipolar transistor device is disclosed that provides protection from ESD. The device functions as a transistor in the active region an ESD event with the current path from collector to emitter and lowering the ESD current density. In U.S. Pat. No. 5,616,943 (Nguyen et al.) ESD protection is described for a mixed voltage circuit and having multiple isolated power supplies. This is accomplished by making use of several overload protection devices. In U.S. Pat. No. 5,677,205 (Williams et al.) an ESD device is discussed which includes a pair of depletion mode MOSFET transistors are connected drain to drain in series with a path from a circuit input terminal to a circuit output terminal. A pair of diodes are connected between ground and the transistors. One diode breaks down during large voltage spikes of short duration and the other diode breaks down during relatively low voltage long duration surges.
When an electrostatic discharge happens, heating takes place in the area of the drain. This is a result of a junction breakdown at the drain which allows a large amount of current to flow. If the current is not spread out across a sufficiently large volume, the resulting heat will not be dissipated and damage to the device will result. As semiconductor devices are shrunk and integrated together in larger and larger quantities, the sensitivity to ESD becomes worse. A way is described in this invention allow small devices and at the same time permit adequate dissipation of heat from an electrostatic discharge.